Defiant Thai opposition protesters stormed the army headquarters and besieged Prime Minister Yingluck Shinawatra's party offices on Friday, intensifying their fight to bring down her government.

Boisterous demonstrators have targeted key government buildings in Bangkok in the biggest street protests since mass rallies in 2010 degenerated into the kingdom's worst civil strife in decades.

The protesters -- a mix of royalists, southerners and the urban middle class sometimes numbering in their tens of thousands -- are united by their loathing of former premier Thaksin Shinawatra.

The controversial former telecoms tycoon was ousted in a coup in 2006 and lives in self-imposed exile, but he is widely believed to be the real power behind the embattled government of his younger sister Yingluck.

Protesters are demanding the end of the "Thaksin regime" and want to replace the government with an unelected "people's council".

Demonstrators forced open the gates of the army headquarters in Bangkok, calling on the military to support their fight to bring down the government.

Thousands of flag-waving demonstrators massed on the lawn inside the army compound in Bangkok's historic district, sheltering under sun umbrellas.

"We want to know whether the army will stand by the people not a dictator," said a protest leader, Amorn Amornrattananont.

Demonstrators also massed outside the headquarters of Yingluck's Puea Thai party, setting up a tense standoff with riot police guarding the building.

The provocative twin-pronged move against key symbols of authority came a day after protesters cut off the electricity to the national police headquarters, ignoring Yingluck's plea for calm after she won a parliamentary no confidence vote.

With their spirits buoyed by free food and a party atmosphere, demonstrators have massed at several locations around the capital, including outside many major government buildings.

Their numbers have fallen sharply since an estimated crowd of up to 180,000 people joined an opposition rally on Sunday.

But turnout is expected to spike again over the weekend as organisers seek a final push ahead of celebrations for revered King Bhumibol Adulyadej's birthday on December 5, which is traditionally marked in an atmosphere of calm and respect.

The carnival-like mood at the rallies masks deep divisions in Thai society that have erupted into political bloodshed on several occasions since Thaksin's overthrow.

While the latest demonstrations have been largely peaceful, a minor clash broke out Thursday between pro- and anti-government supporters in the province of Pathum Thani on the northern outskirts of Bangkok, police said.

"Two anti-government supporters suffered minor injuries. They might have been hit with a wooden stick," said provincial police commander Major General Smithi Mukdasanit.

A minor confrontation between the two sides was also reported in the northeastern province of Mahasarakam although nobody was injured.

Thaksin remains a hugely divisive figure seven years after he was deposed by royalist generals. Pro-Thaksin parties have won every election for more than a decade.

He is adored by many of the country's rural and urban working class but hated by many southerners, middle-class Thais and the Bangkok elite, who see him as corrupt and a threat to the monarchy.

In a televised address Thursday, Yingluck urged demonstrators to call off their protesters and said the government did not want confrontation.

But a defiant rally leader, Suthep Thaugsuban, rejected any suggestion of talks in a tub-thumping late night speech that set a fresh deadline for the demos to reach their climax.

"Yingluck said the government can still govern, can still work -- I want to say that they will only be able to work for a few more days, then we will not let them work anymore," he told several thousand supporters in Bangkok.

The protests snowballed after the ruling party tried to introduce an amnesty that could have allowed Thaksin's return from self-imposed exile, and the rallies have continued despite a Senate move to reject the bill.

Yingluck on Monday ordered special security measures to be expanded to cover all of the capital, although she has ruled out using force against the demonstrators.